This invention relates to a video signal transmission system, and more particularly a video signal transmission system wherein a television video signal is transmitted by inverting its polarity with a pseudo-random code (PN code) in unit of a horizontal scanning period, and the transmitted signal is restored and reproduced on the receiving side.
In a fee charging television program distribution system utilizing a satellite and a fee charging wired/wireless television broadcasting system, it is necessary to use such a transmission system wherein the television video signal is transmitted after being subjected to a special waveform conversion processing so as not to be readily received by an ordinary receiver set in order that only a contracted receiver can restore the received signal for reproducing a high quality picture. To assure the transmission system of this type, there are available a number of basic processing methods and composite processing methods comprising the basic processing methods in combination.
Among relatively simple basic processing methods, a method may be mentioned wherein the polarity of the white and black levels of a video signal is inverted by taking an intermediate gray level as a reference. According to another method, only the synchronizing signal is inverted and expanded by using a pedestal level as a reference and then transmitted. According to these prior art methods in which the polarity is inverted, the horizontal synchronizing signal usually utilized as the reference level at the time of regenerating a DC component and the video signal are processed independently. As a result, a level difference is caused in the restored video signal owing to variations in the inversion reference level caused by a DC drift of an inversion processing circuit and owing to a difference in the reference levels on the transmission side where the signal is subjected to an inversion processing and on the receiving side where the signal is restored. Therefore, when the inversion processing is performed in unit of scanning line, a stripe shaped tone difference would occur in the picture, thus failing to stably reproduce a high quality picture.
Typically, for restoring the polarity inverted television video signal and regenerating the DC component thereof a feedback compression type clamping circuit has been used. However, the prior art feedback compression type clamping circuit can not be applied as it is to such a signal inclusive of the horizontal synchronizing signal having indefinite polarity for each horizontal scanning line. Even when the inverted polarity is suitable for a restored video signal, it is impossible to eliminate low frequency variation components, failing to provide perfect regeneration of the DC component.